Changes since SOA version
IXFR is a zone transfer request that returns changes since a specific SOA version. Enter the SOA version to request changes since then.
Not an apex domain
The domain 9s1ld2jt4vcg129or1fm5iu5djk0mnmi.smtp.dk is not the root of its zone.
Instead, try zone transfer on smtp.dk .
Instead, try zone transfer on smtp.dk .